Geostationary satellite systems have already demonstrated the ability to perform many communication, meteorological, and scientific missions when launched by expendable launch vehicles. The NASA Space Transportation System (STS), the Space Shuttle, offers the opportunity for a significant improvement in the performance and cost of satellites designed to take advantage of its capabilities. It therefore can make current applications more profitable and new uses economically attractive.
All satellite systems require launch vehicles, and all current launch vehicles are expended after delivering their spacecraft payload into orbit. The NASA Space Shuttle by contrast, introduces a new concept of being recoverable and reusable. Tests have proved that the Space Shuttle can be piloted like an airplane after re-entering the atmosphere from space.
All geostationary satellites so far have been launched by Thor Delta, Atlas Agena, Atlas Centaur, or Titan IIIC launch vehicles. Now, however, the spacecraft designer has the choice of six launch vehicles: Thor Delta (2914, 3914, 3910 PAM), N-Rocket, Atlas Centaur, Ariane, Titan IIIC, and the Space Transportation System. The N-Rocket is being developed by Japan's NASDA (National Space Development Agency) and the Ariane by Europe's ESA (European Space Agency). The United States plans to phase out the Thor Delta, Atlas Centaur, and Titan IIIC as the STS becomes operational in 1980. The returnable and reusable Space Shuttle offers the challenge and opportunity to geostationary spacecraft designers to make the best use of it.
The Space Shuttle will orbit Earth at a nominal 160 n.mi. with an orbit inclination of 28.6 deg when launched due east from Florida. A geostationary satellite must orbit at approximately 19,300 nautical miles north of the equator. The STS therefore needs an upper stage to launch geostationary satellites. The upper stage requirements are optimally satisfied by two propulsion impulses. At the time of an equatorial crossing, the first impulse imparts a velocity increment of approximately 8000 fps at the perigee of elliptical transfer orbit. At an appropriate apogee of the transfer orbit the second impulse imparts a velocity increment of 6000 fps, both circularizing the orbit and removing the inclination.
The central challenge in using the STS for the launch of geostationary satellites lies in finding the combination of upper stage and satellite geometry and functions that minimizes overall mission cost.
Upper-Stage Alternatives: The first upper-stage concepts considered for the STS completely separated orbit-injection functions from subsequent orbit-control requirements. These bulky and expensive "stand alone" upper stages obscure the basic economic advantages of the STS.
It soon became apparent that the STS could take advantage of the geostationary orbit-injection scheme pioneered by Syncom III in an historic Delta launch in 1964. This early capability was achieved by incorporating the apogee-boost motor within the satellite, thus permitting separation from the Delta at perigee injection. The perigee boost itself was provided primarily by the Delta's unguided, spinning upper stage. Since the apogee boost does not parallel the perigee boost, it was necessary to reorient the spacecraft spin axis before apogee-motor firing. This was done by the control and attitude sensing system required for Syncom's operational mission. The use of the satellite's telemetry and command system and communication repeaters to determine the transfer-orbit parameters via ground tracking permitted the selection of an apogee-motor firing time and attitude that minimized the effects of transfer-orbit injection errors. The subsequent launch of some 50 geostationary satellites by the Delta and Atlas Centaur boosters brought refinements but no basic changes to this technique.
NASA adopted the Spinning Solid Upper Stage (SSUS) as its preferred method of launching via the STS geostationary spacecraft previously designed for expendable launch vehicles. These upper stages are now known as SSUS-D for Delta and SSUS-A for Atlas Centaur replacements. The USAF has elected to retain the independent upper stage concept used in the Titan IIIC for its STS launches of spacecraft to high-energy orbits. It will use a pair of guided and controlled solid-propellant rocket stages known as the IUS (for Interim Upper Stage).
The STS/SSUS does offer a lower launch cost than the expendable launch vehicles for organizations unwilling or unable to depend exclusively on Space Shuttle operational availability. The accommodation of transition spacecraft in the Space Shuttle, while retaining their ability to be launched on expendable launch vehicles, has proven to be achievable. In the case of new spacecraft, the dual capability can readily be incorporated in the initial design.
Even for transition spacecraft, the STS reduces launch costs significantly. The STS economics clearly support the desirability of incorporating dual capability into transition spacecraft, permitting Space Shuttle launch while maintaining expendable-launch-vehicle backup.
The transition spacecraft, attractive as they are, do not represent optimum designs for the Space Shuttle, for two reasons. First the restriction on diameter imposed by the expendable launch vehicles makes them longer than otherwise necessary. The SSUS-D and SSUS-A configurations, for example, require a quarter, and a half, respectively, of the Space Shuttle payload bay volume, but use a much smaller fraction of the available weight. The dual compatibility thus levies a penalty of higher than necessary launch cost of the Space shuttle. As another disadvantage, such dual-launch spacecraft do not have as much space for mounting antennas, cameras, and scientific instruments as would one designed only for Shuttle launch.